A jig or fixture is used to speed up and simplify repetitive machining of like parts. The jib serves to hold and accurately locate the part being machined as well as to accurately guide the machine tool. In the case of a drill jig, the jig is used to hold and accurately locate the part being drilled upon and as a guide for the drill or drills whereby like holes may be drilled at similar locations in like parts in repetitive drilling operations.
Various kinds of jigs are known. One kind is characterized by a plurality of interchangeable and interconnectable jig building elements that may be assembled in a number of ways to obtain a desired jig configuration for the particular part to be machined. These building elements comprise base plates, sub-bases, angle plates, V-blocks, bushings and the like of various sizes and shapes which are machine finished so that they may be assembled in a variety of different combinations and configurations. A representative jig of this kind is shown in U.S. Pat. No. 2,676,413.
Another kind of jig, principally a drill jig, is characterized by a drill bushing top plate which is moved towards and away from a part supporting base or table by a quick acting clamp and lock mechanism. the drill bushing top plate may have located therein one or more drill guide bushings as the needs of the work require and most often are interchangeable so that the same base and clamping mechanism may be employed with different top plates for other jobs. Typically, the bottom surface of the bushing top plate and the opposed top surface of the base are precision machined flat, and the bushing top plate is guided by one or more upright guide posts so as to maintain the machined surfaces parallel for proper clamping of the part therebetween. The part may be directly supported on the top surface of the base but, most often, it is necessary to use a subbase on the base to elevate the part to a position engageable by the bushing top plate within its usually limited range of clamping movement. Locating elements and stops may be attached to or provided in the bushing top plate or the sub-base for proper positioning of the part within the jig. It is conceivable that locating elements may be attached to or provided in the top surface of the base but this may preclude or limit its usage in different jobs. For a representative drill jig of this kind employing a rack and pinion clamping mechanism, reference may be had to U.S. Pat. No. 2,414,348.
Still another kind of drill jig employing a piston-cylinder clamp mechanism, is disclosed in applicant's own U.S. Pat. No. 3,313,188. This drill jig is characterized by an L-shaped bushing plate and base which permit drilling in one or two planes. The sides of the bushing plate form a right angular corner and respectively have planar bottom and side clamping faces which are precision machined square. In an alternative arrangement, the bushing plate further may have a third side which permits drilling in still another plane.
In drill jigs of the aforedescribed and other types, precision in locating the drill guide bushings is important to proper drilling of the holes in the part held and located in the jig. It is equally important to locate precisely the guide post holes to enable the desired movement of the bushing plate without binding while maintaining the desired tolerances between the bushing plate, guide posts and jig base. Generally, the guide post holes are pre-drilled by the jib manufacturer who supplies the assembled jig to the part manufacturer. The part manufacturer, as needed, drills the drill guide bushing holes in the bushing top plate as required for the particular part to be drilled upon in the jig.
Heretofore, it was necessary for the part manufacturer to use the guide post holes in the bushing top plate as a reference in laying out the locations of the drill guide bushings. This normally required finding the centers of the previously drilled guide post holes, which generally cannot be easily, quickly and precisely done. To simplify matters, templates have been used. The location of the drill guide bushings are first layed out on the template which then is placed on the bushing top plate and aligned with the guide post holes therein for transfer of the drill guide bushing locations to the bushing top plate. This procedure also has proved to be relatively difficult and time consuming.